Exciton Trapping at Shape-Persistent Molecular Nanotubes

We report a series of shape-persistent molecular nanotubes with top rim connectivity traversing from an all-meta- (m₄) to an all-para-phenylene (p₄) bridged species, including all possible members in between them. Single-crystal X-ray diffraction (SCXRD) and microcrystal electron diffraction (MicroED) data show a large torsional angle for meta-phenylenes relative to para-phenylene rings. Density functional theory (DFT) calculations reproduce the experimental torsional angles and also establish a correlation indicating a gradual increase in strain energy from m₄ (∼31 kcal mol^-1) to p₄ (∼90 kcal mol^-1). Structural transitions from m₄ to p₄ lead to additional correlations such as a shift in the lowest absorption wavelength from 330 to 394 nm, a sizeable red shift in the maximum emission wavelength from 444 to 546 nm, and a decrease in fluorescence quantum yield from 0.76 to 0.20, respectively. Time-dependent (TD)-DFT analysis of the relaxed excited state (S_1') geometry shows a progression of exciton delocalization as para-phenylenes are introduced into m₄ en route to p₄, while the overall molecular size remains constant. This effect is directly related to increased π-conjugation within the nanotube's top-segment and demonstrates how exciton trapping can take place without changing the nanotube's physical size, e.g., diameter and length.